My list:
rack of some sort (I can get a sweaty back with a backpack on just by thinking about exercise).
Chain guard for the trouser cuff protection.
lights that are easily removable. either fully, like those elasticated lezyne (and copies) ones, or where you can buy the mount again separately should the worst happen.
26 inch wheels – you can probably tube and tyre it for free from most MTBers shed pile.
If you dont want single speed, some ancient 1×7 might do you for gearing and will last forever.
If this is a true 1-less-car, mudguards. If you are going to look out the window and decide between car and bike, I wouldn’t bother.
Oh and saddles – clearly no need for long distance comfort, but 2 points of caution.
Cheapo foam filled saddles with a cut or hole in the covering? – the foam stays wet for days and will give an awkward wet spot when you sit on it and compress it, even if the top felt dry to the touch.
Anything with seams/stiching on the top can ruin a nice/expensive pair of jeans in short order.